124 



parts tin, and 1| arsenic. I suppose, with others, that, 

 if the metal be granulated, by pouring it, when first 

 melted, into water, and then fused a second time, it 

 will be less porous than at first. 



In this process, whatever metals are used, and in what 

 proportions soever, the chief object is, to hit on the exact 

 point of saturation of the copper, &c. by the tin. For, 

 if the latter be added in too great quantity, the metal 

 will be dull-coloured and soft; if too little, it will not 

 attain the most perfect whiteness, and will certainly tar- 

 nish. It is too late to discover the imperfections of the 

 metal, after the mirrors are cast and polished; and no 

 tokens given of them (that I know) are sufficiently free 

 from ambiguity. But I observed the following, which 

 proved, in my trials, at first view, indubitable marks of 

 the degree of saturation; and I think it fit to describe 

 them particularly, as they have not, to my knowledge, 

 been noticed by others. 



When the metal was melted, and before I poured it 

 into the flask, I always took about the quantity of an 

 ounce of it, with a small ladle, out of the crucible, and 

 poured it on a cold flag; and observed the following 

 appearances. 



First. If the metal assumed, in cooling, a lively blue, 

 or purple colour, commonly intermixed with clouds, or 

 shades of green or yellow; and if, when broken, the 

 face of the fracture exhibited a silvery whiteness, as 

 bright and glistenitng as quicksilver, without any ap- 

 pearance 



